Patent GB-849643-A (GB)
Title: Improvements in or relating to hyperbolic radio navigation systems
Inventor: ROBERTS GRAHAM ESTYN
IPC: G06F7/548
Filed: 1958-06-27
Abstract: 849,643. Photo-electric computation. DECCA RECORD CO. Ltd. June 25, 1959 [June 27, 1958], No. 20766/58. Class 40(3). [Also in Group XL(c)] A computer for use in conjunction with a hyperbolic radio-navigational aid converts the hyperbolic, path difference, co-ordinates obtained by the aid into cartesian co-ordinates. This is effected by providing means for feeding an assumed set of cartesian co-ordinates into triangle solving means which determine the distances from the assumed position to the stations of the navigational aid, means for subtracting these, differences in pairs to obtain the hyperbolic co-ordinates of the assumed position, and means for comparing the true and assumed hyperbolic co-ordinates to obtain error signals for the correction of the assumed coordinates. The computer is arranged to operate so as to successively correct the assumed cartesian co-ordinates until these correspond to the required degree of accuracy, to the hyperbolic co-ordinates supplied by the navigational aid. Except when the apparatus has just been put into operation the assumed position is the position determined immediately previously. Analogue computing apparatus may be used but in the embodiment described use is made of apparatus handling the information in binary digital form. In this apparatus two shafts 11 and 12, positioned by radio navigational receiver 10 in accordance with the hyperbolic co-ordinates of the craft position, drive analogue to digital converters 13 and 14 at the computer input. The output of the computer may drive automatic chart display 15 through gear-box 18 which may operate to change the cartesian co-ordinate axes and relative scales. For the operation of the computer itself it is preferred to use cartesian co-ordinate axes with the master station as origin, one axis parallel to the line joining the slave stations, and the other perpendicular thereto. The output cartesian co-ordinates with respect to these axes are derived from the computer over shafts 16 and 17 which are driven by servo-mechanisms 19 and 20. The shafts each carry one of the analogueto-digital converters 21, 22 whose binary digital outputs are used as the input to the computer representing the assumed cartesian position. This assumed position information is fed to each of triangle solving means 23, 24 and 25. Unit 24 calculates the distance of the assumed position from the master station and units 23 and 25 the distances to the slave stations. Subtraction units 26 and 27 determine the hyperbolic co-ordinates of the assumed position and these are compared with the actual hyperbolic co-ordinates in units 28 and 29 to give error signals fed to correction unit 30. The device is fed with assumed position information from converters 31 and 32 on shafts 16 and 17 respectively and uses this to determine appropriate constants with which to process the incoming error signals to give correction signals fed out to servo-mechanisms 19 and 20, to bring the assumed position co-ordinates into closer correspondence with the actual hyperbolic co-ordinates. An embodiment of triangle solving means suitable for use in the computer is shown in Fig. 2. Input unit 50, not necessary in the case of the master station which is at the origin, determines a and b, the differences between, the corresponding co-ordinates of the assumed position and of the navigational aid station. h, the distance of the position from the station, is thus given by h<SP>2</SP> = a<SP>2</SP> +b<SP>2</SP>. It is preferred to solve this equation in a modified form got by the following re-arrangement:- To perform the solution a value for the length h is assumed, the smaller of the other two sides of the triangle is regarded as a and the value of b computed from the last equation above and compared with the actual value of b. The error signal obtained is used to correct the assumed value of h so that after a series of approximations h is correct to the required degree of accuracy. The assumed value of h is obtained from unit 51 and is fed to subtraction unit 52 and adding unit 53 along with the value of a from unit 50. The resulting values of (h -a) and (h + a) are fed to comparators 54 and 55 used to control the counting of pulses from the logarithmic calculating device. This device consists of a constantly rotated drum 40 which has engraved lines marked round its periphery on four circular tracks 42, 43, 44 and 45. A separate one of photocells 46, 47, 48, 49 is arranged opposite each track and produces output pulses corresponding to the marks passing it. Two of the drum scales 42 and 45 are linear whilst the other two 43 and 44, are corresponding logarithmic scales such that the number of markings in any given sector corresponds to the logarithm of the ratio of the numbers represented by the markings at the two ends of the corresponding sector on the linear scale. Commencing from an arbitrary datum on linear scale 42 pulses from photo-cell 46 are counted in counter 56 until the binary digital output is equal to (h - a) as determined by comparator 54. During this period pulses from photo-cell 47 are fed through switching unit 57 and counted by counter 58. When comparator 54 determines that coincidence has been reached it actuates switching unit 57 in such a way that only alternate input pulses are passed to counter 58. Counter 58 continues counting in this way until the output from counter 56 reaches the value (h + a) as determined by comparator 55. The counter 58 is then switched off by the output from comparator 55 applied to switch 57 and stores a count corresponding to log b. b is obtained by counting pulses from scales 44 and 45. Starting from a fresh datum position pulses derived from scale 44 are counted at 60 until comparator 59 determines that the same count has been reached as that stored in counter 58. The comparator then terminates the count made during the same period of pulses derived from linear scale 45 in counter 61. The resultant stored count in counter 61 is thus the value of b corresponding to the assumed value of h. It is compared in unit 62 with the value of b derived from unit 50 and a correction signal fed back to unit 51 to correct the assumed value of h. The drum 40 need only have two scales and when the log b count has been reached on counter 58 it can be arranged to count down to zero whilst a further count (corresponding to b) is made by counter 56 for application to comparator 62. In another arrangement using only two scales counter 58 remains at zero until (h - a) is counted by counter 56, then counter 58 is freed to count every marking on scale 43 until the scale 42 pulse output reaches the value b as given by unit 50. A count down of the number in counter 58 is then effected until (h + a) is reached on the natural number scale. The remaining count (if any) on counter 58 is then a measure of the error in the assumed value of h and can be applied to effect a correction. A single drum 40 may be provided with sets of scales for use by all the triangle solving means 23, 24 and 25 or, alternatively, a single set could be used by all the means on a time sharing basis the outputs being stored as necessary. As it cannot be arranged for the logarithmic scale or scales to be infinitely long the natural number scale cannot be taken down to zero. This means that the computer will not function in regions within a predetermined distance of each transmitter. However, although the ratio between the maximum and minimum numbers represented determines the length of the scale, the scales may be used to represent a fraction of their nominal number values. Thus if scale 42 extends from n to 2n it may be used for reading from n/2 to n by counting only alternate output pulses and in general from n/m to 2n/m by counting only every xth pulse where m = 2 <SP>x-1</SP> For this purpose switching devices 70 and 73 controlled by control unit 72 driven by gearing 71 from the drum drive may be provided as indicated. Specifications 620,479 and 665,135 are referred to.